This invention relates to an improved dual motor, single-axis position and velocity servo system for a patient-supporting table that is used to position a patient in an x-ray beam in computed axial tomography and in computed projection radiography apparatus, for example.
As is known, in one type of computed axial tomography apparatus, an x-ray source is mounted on a scanner base that is journaled for rotation about a nominally horizontal axis on a tiltable gantry. A multiple cell x-ray detector is mounted on the scanner base on the opposite side of the axis from the x-ray source. The x-ray beam emanating from the source is collimated into a fan-shaped configuration that spreads over the circumferential length of the detector and is thin in the direction to which the rotational axis of the scanner base is perpendicular. The patient who is to be examined is customarily supported on an x-ray-transmissive table top or cradle in coincidence with the rotational axis of the scanner. The cradle must be supported from a floor-mounted base in a fashion that allows it to be moved in what is called the axial or longitudinal direction relative to the base so that the x-ray beam may be caused to penetrate successive layers of the body. The base also includes an elevator mechanism for adjusting the vertical position of the cradle relative to the gantry. As is well known, the computed axial tomography process requires orbiting the x-ray source and detector jointly about the patient so that the detector will be able to produce analog signals representative of x-ray beam attenuation by the patient for a multiplicity of x-ray source and detector positions that are attained in a 360.degree. orbit in one direction or the other. As is well known, the signals representative of beam attenuation are variously processed with a computer system that yields digital data representative of the intensity of the picture elements that comprise cross-sectional image of the body layer that has been scanned. The picture element data is converted to analog video signals and is used to display the image on the display screen of a video monitor.
The gantry allows the scanner to be tilted so that the fan-shaped x-ray beam, instead of being projected perfectly vertically, is projected at an angle relative to the horizontal axis to permit imaging angular rather than vertical body layers. Pre-existing computed axial tomography apparatus typically provided for tilting the plane of the fan-shaped beam through an angular range of about 15.degree. from either side of vertical. Since the gantry is a large upright structure, when tilted through a small angle such as 15.degree., its bottom swings out toward the base on which the axially movable patient-supporting cradle is mounted, but the base can be set far enough away from the gantry to avoid having the latter strike the base. This is so because the x-ray-transmissive cradle supports the patient in cantilever fashion from the base. However, in a more advanced computed axial tomography apparatus design, tilting of the gantry by as much as 20.degree. from either side of vertical has been provided for. Thus, the base that supports the cradle must be set farther away from the gantry to avoid interference by the base when the gantry is tilted through the larger angular range. Hence, in the new design, it became necessary to increase the distance through which the patient is translated axially relative to the base. It would be possible to lengthen the cradle so a major part of the patient could be advanced into the x-ray beam but, since the patient would be supported in cantilever fashion, intolerable deflection of the cradle would result. The alternative, disclosed and claimed in the aforementioned patient application, that was adopted is to make the patient-supporting table assembly in two sections comprised of an intermediate support or carriage that moves relative to the base and a patient cradle mounted on the intermediate support for moving axially relative to it. The intermediate support and cradle are driven by individual servomotors which are sequentially activated and controlled so as to transport the patient-supporting cradle at a constant overall velocity and, if desired, to stop the movement at predetermined positions to enable imaging of desired anatomical features.
In connection with the computed projection radiography method, using the apparatus described briefly above, the x-ray source and detector are held in a fixed position rather than being orbited to perform a scan as in the computed axial tomography method. In this method, the patient must be advanced through the fan-shaped x-ray beam at a very constant velocity for undergoing a line-by-line scan with a fan beam that is about 1.5 mm thick, for example. As the patient is being advanced, the x-ray detector cells yield analog signals corresponding to x-ray attenuation at closely successive positions of the patient on a line-by-line basis, and the resulting attenuation data is stored until the length of the body which is of interest has been scanned. A computer then uses the attenuation data to produce digital data representative of the intensities of the picture elements for all scan lines, and these signals are used to drive a video monitor which displays a visual image corresponding to the projected x-ray image. In effect, the computed projection radiography method yields a visual x-ray image that is comparable to the image obtained with ordinary radiographic film but with greater contrast than is obtainable with film, because the dynamic range of the x-ray detector is usually greater than that of film.
The inventive x-ray table disclosed herein includes unique structural features which enhance the operating characteristics thereof and which provide desirable safety features as compared to conventional patient-positioning devices. One structural feature is a zero-backlash longitudinal friction drive for moving a patient cradle so as to enable accurate positioning of the patient for scanning. A safety feature is provided in that the driving friction between the drive unit and the patient cradle is proportional to the weight of the patient. In this manner, the driving force can never exert more pressure to the patient than the coupling friction, unlike the torque of the drive, as in most systems, which is designed for the heaviest load. This drive also incorporates a pivot which proportions the weight of the patient between a drive roller and an idler roller and provides compensation for unavoidable flex in the patient cradle. This system also eliminates the need for expensive and noisy gear rack, ball-and-screw drive, belt, or chain drives used in the past to achieve zero backlash. Another safety device which is provided senses any force more than 25 lbs, for example, exerted on either the patient or patient support in a longitudinal direction, and disengages the servomotors. The force may be exerted by the drive, gantry, or other outside agency. Longitudinal position accuracy is obtained by the use of an encoder attached to the cradle by a closed-loop cable system which records the total patient longitudinal motion, even though there is a two-stage drive. Since the encoder is not placed in the drive mechanism, but senses actual cradle motion, it can very accurately indicate cradle position and speed. Another important feature of the improved x-ray table is the use of a pair of gas springs mounted at the gantry end of the x-ray table to exert an upward force on the front end of the table to compensate for the cantilevered load which is exerted as the patient is extended in the longitudinal direction. The upward force exerted by the gas springs reduces the loading on the elevator drive mechanism and permits smooth and quiet operation. The gas springs also compensate for the weight of the elevation mechanism and are less bulky and complicated than previously used counterbalance devices.
It is, therefore, an object of the invention to provide an improved x-ray table which enables the accurate positioning of the patient for scanning.
It is another object of the invention to provide an x-ray table having features to enhance the safety of the patient.
It is still another object of the invention to provide an x-ray table to very accurately indicate the position and speed of the patient support so as to ensure high image quality.
It is a further object of the invention to provide an x-ray table which is less bulky and complex and which operates in a smooth and quiet manner.